I'll make an attempt.I was introduced to existentialism decades ago by an uncle. It registered but didn't change anything.
A couple of years ago I am out in the desert. I look up. I see a long band of clouds. But there is no humidity out here. Then I realize it is the milky way. Our galaxy. And I'm thinking, this hasn't changed for a very long time. Now Jesus was out in the desert. He saw the same milky way. But he had no idea whatsoever that he was looking at our spiral galaxy edge on from the perspective of being in a solar system somewhat on the outskirts of our galaxy. Therefore the claim that he is second person of the trinity, co-creator of the universe is absurd. Mythology. So the Christian belief system is in error. But so is the Muslim. So is the Hindu etc. etc.
All belief systems are valid if you adhere to them. But they are relative. So what is behind the belief systems? What is at a deeper level?
I exist. I am a human who exists in this time period of history, on this planet, who is capable of many belief systems. Pick one. They all work. But the ultimate ground of being is our existence.